The Fischer-Tropsch (FT) process involves the conversion of carbon monoxide (CO) and hydrogen (H2) to a hydrocarbon (CnH2n+2) and water (H2O) and is employed for the manufacture of liquid hydrocarbons from natural gas, coal, or biomass energy sources. The FT process is most frequently carried out in a fixed bed reactor and requires a catalyst that is normally an active metal or combination of metals that are supported on a refractory oxide.
It is desirable to employ a catalyst that is selective for preparation of hydrocarbons larger than butanes, yet most catalysts form significant amounts of methane, other gaseous hydrocarbons, and waxes. Typically very active catalysts used at low reaction temperatures display better selectivity for liquid alkanes.
Control of the catalytic structure to an eggshell design where the active metals are situated near the particle's surface is preferred to promote selectivity to liquid alkanes as it has been shown that catalytic sites residing in deep small pores of a catalyst particle appear to promote methane formation, presumably due to the greater rate of diffusion of H2 than CO into these deep small pores.
A number of synthetic strategies have been employed to achieve particles with the eggshell architecture, but it is often difficult to control the depth of the catalyst region. Behrmann et al., U.S. Pat. No. 4,962,078, discloses dispersing cobalt as a thin catalytically active film upon a particulate titania or titania-containing support by spraying a solution of a cobalt compound onto titania or titania-containing particles that are preheated and maintained at a temperature of at least 140° C. during spraying. Mauldin et al., U.S. Pat. No. 4,977,126, discloses impregnating and dispersing, as a film or layer, a catalytically effective amount of cobalt on the periphery of a particulate porous inorganic oxide support by spraying a bed of fluidized support particles with a liquid containing a cobalt compound while at 50 to 100° C. Iglesia el al., U.S. Pat. No. 5,036,032, discloses rim type catalyst where support particles are impregnated with a molten cobalt compound at a temperature sufficiently near the melting point to ensure high viscosity of the melt and minimize diffusion into the particles' pores. Peluso et al., Chemical Engineering Science 56(2001) 1239-45, discloses bathing fritted filter supported 1.81 mm silica particles with an aqueous metal impregnation solution for a period of 5 to 60 seconds followed by rapid removal of the solution and transferring the particles to a fluidized bed of 60 μm sand at 90° C. before the particles are calcined at 400° C. Kuipers et al., U.S. Patent Application No. 2008/0255257, discloses a method where a particle having a homogeneous distribution of the catalytic metals are treated with an organic acid for periods of 5 minutes to 500 hours (preferably 10 to 50 hours) at temperatures below 150° C. to result in migration of the metals toward the surface of the particle to yield an eggshell structure.
There remains a need to prepare a FT catalyst with an eggshell structure that has a high selectivity for hydrocarbons of a desired molecular weight, yet is readily prepared in a simple, reproducible, and controllable manner.